(1) Field of the Invention
The present invention generally relates to heads-up and/or head mounted display devices. This invention more specifically relates to a novel reflective thin film image combiner for the aforementioned devices.
(2) Background Information
Helmet mounted displays and windshield mounted displays (hereafter referred to as heads-up displays or HUDs) are increasing in importance in both military and civilian environments. For example, in a military setting, a heads-up display presents valuable information to pilots by superposing virtual images of data information over the pilots' normal field of vision. The flight parameters of the aircraft, navigational displays or any other relevant information may be viewed without the pilot moving his/her head, thereby generally improving flight performance and safety. HUDs are also finding wider usage in civilian transportation, with windshield displays becoming popular in many automobiles. Further, HUDs are beginning to find a wide range of application in the entertainment markets, such as in videos, video games, and virtual reality.
A general and basic design for a HUD 20 is shown in FIG. 1, in which a transparent substrate 22 (e.g. a visor or windshield) includes an image projection system 26 and an image combiner 24, which together superimpose display information over the user's field of view. Image projection system 26 projects optical information (generally in the form of a virtual image) towards image combiner 24, which is superposed on transparent substrate 22. The virtual image is then reflected towards the user. The user may, therefore, view the information provided by image projection system 26, without turning away from his/her normal field of vision. Image projection system 26 generally includes a cathode ray tube (CRT) or a liquid crystal display (LCD) in combination with imaging optics (e.g. mirrors and lenses).
In order to minimize any undesirable distortion to the user's target detection distance and/or color perception, image combiner 24 is generally designed as a narrow spectrum reflector, (i.e. it reflects light only in a narrow wavelength range and transmits substantially all light at other wavelengths). Existing image combiner 24 technologies generally include semi-reflective thin-film deposition coatings, holographic film coatings (see for example Moss in U.S. Pat. No. 4,880,287), or cholesteric liquid crystal based reflective coatings (see for example Berman in U.S. Pat. No. 4,900,133, which is fully incorporated herein by reference). Furthermore, image projection system 26 may be designed to project light in a narrow band, matching the reflective band of the image combiner 24.
However, despite these advancements, state of the art HUDs are generally limited in that image combiner 24 is passive (i.e. the reflectivity of the coating is constant after fabrication). If, for example, the light intensity of the outside environment is bright the user may find it difficult to read the displayed information over the bright background. Conversely, if the outside environment is dark, and the displayed information is relatively bright, the user may observe reflections of his or her face (as well as other distracting glare) superimposed on the field of view. Prior art HUDs attempt to overcome this problem by adjusting the brightness of the CRT or LCD panel in projection system 26. However, the light intensity range achievable from the projection system 26 tends to be limited and may not always satisfy practical requirements. Moreover, for some applications, the user may only be required to view the display at intermittent periods. One may turn off image projection system 26 to eliminate the display image, however, image combiner 24 remains in the user's field of view and may adversely affect the perception of outside objects, particularly since image combiner 24 is not generally perfectly transmissive.
Therefore, there exists a need for an improved HUD including an improved image combiner 24.